1. Field of the Invention
The invention relates to oil well logging apparatus known as nuclear magnetism logging equipment requiring periodic high-power transmissions with an extremely short duty cycle from surface sources of alternating current power to downhole polarizing coils.
2. Description of Prior Art
Nuclear magnetism logging equipment comprises surface control apparatus and downhole apparatus in a sonde. The sonde and surface apparatus are in electrical communication by means of electrical cables. Alternating current, generated in the surface apparatus, is applied via a cable to the sonde apparatus where it is rectified and applied to a coil, producing a magnetic field in the borehole and formation. This field, called the "polarizing field," serves to polarize the alignment of protons in water, oil and gas. After the polarizing current has been applied long enough to build up adequate polarization, the current is rapidly removed, leaving the protons to precess in the earth's magnetic field and, in so doing, to induce a damped sinusoidal signal in the same coil used to induce the polarization. The signal has a frequency of about two thousand hertz. The signal is damped with a time constant that is frequently about 50 milliseconds, the decay time depending on various factors.
During the signal-receiving time, the coil is connected to an amplifier and the signal sent to the surface apparatus via an electrical lead. At the surface, the signal can be displayed in various ways. In particular, the signal amplitude can be recorded on the log as an indication of the amount of hydrogen in formation fuilds in the reservoir rocks. The signal has several characteristics that can be observed and interpreted. Examples of these characteristics are signal amplitude, signal decay time and the relationship between signal amplitude and polarizing time. The induced signal amplitude, at the instant precession begins, is directly proportional to the number of protons per unit rock volume in the water, oil and gas in the vicinity of the borehole. However, the transition from the removal of the polarizing field to signal receiving takes a finite amount of time, and a time delay occurs before the signal may be observed.
The operational requirements of the downhole apparatus cause severe specifications for the surface power generation and transmission circuits to the downhole equipment. Because of the low signal level of the precessing protons, alternating-current power transmission to the sonde must terminate during the signal-receiving time.
At each depth location of the sonde, one measurement requires a relatively long time for polarization ad signal reception. It is necessary to perform the polarizing-signal reception cycle as often as possible in order to maximize the number of received signals, yet minimize the time the sonde is in the borehole. Power transmissions are required for extremely short duty cycles, occasionally as short as 100 milliseconds.
These requirements put severe electrical requirements on the surface electrical generating equipment when turned on and off with extremely short duty cycles to magnetic loads. Operation of a nuclear magnetism logging system with short duty cycle switching of alternating current generators to iron-core transformers causes extremely high current surges which must be supplied by the generator.
Prior art apparatus and methods for switching alternating power sources to iron-core transformer loads have recognized the benefit of opening and closing a switch between the load and the source at a zero amplitude point of the alternating current waveform. Switching at a time when the current through the transformer is zero helps to eliminate high current transients through the transformer. Early prior art power switches have used a simple zero crossing mechanism which opens and closes as the alternating current waveform crosses zero.
Later refinements to the zero crossing switches art have seen the addition of means for storing information regarding the turnoff polarity of the previous alternating current waveforms. These switches insure that the source of alternating power is connected to the magnetic load with a polarity opposite that of the previous disconnect polarity. For example, the prior art "zero crossing switches with memory" typically disconnect the power source at a zero amplitude point after a negative half-cycle, and apply the current waveform for the next transmission at zero amplitude, but at the point before the positive-going half-cycle of the alternating waveform. These prior art switches typically turn on with the same polarity for each power transmission and turn off each time with a polarity opposite the turn-on polarity. The zero crossing switches described above are manufactured commercially by the Hamlin Corporation and the Crydom Company. These switches are described in technical literature published by the Hamlin Electronics Inc., Lake and Grove Sts., Lake Mills, Wis. and by the Crydom Division of International Rectifier, 1521 Grand Ave., El Segundo, Calif.
These prior art switches function in a satisfactory manner when the transmission duty cycle is relatively long (e.g., the time during which the power source is connected to the magnetic load is long compared with the transient time associated with the iron-core transformer). Transformer transient time is the time required for the transformer to reach steady state voltage and current conditions after power is applied to it from a transformer level of magnetization remaining when it was previously turned off. These prior art switches typically cause extremely high transformer current surges which must be supplied by the power source when the duty cycle time of applying the power to the iron-core transformer is short compared with the transformer transient time.
It is an object of this invention to provide a nuclear magnetism logging system having power switching circuitry capable of providing power transmissions for extremely short duty cycles, as short as 100 milliseconds.
It is another object of this invention to provide nuclear magnetism logging apparatus and methods for switching a surface source of alternating current power to a downhole sonde while minimizing the transient current surge requirements of an ironcore transformer under short duty cycle conditions.